This invention relates to improvements relating to a fuel rod for use in nuclear reactors, and more particularly to a nuclear fuel rod of the type wherein a stack of fuel pellets is contained in a sheath.
A nuclear fuel rod now being used widely has a construction as shown in FIG. 1. Thus, a number of fuel pellets 2 are stacked in a sheath 1 made of Zircaloy (registered trade mark) to constitute an effective fuel portion 3 and a plenum chamber 4 formed above the effective fuel portion 3 for containing a coil spring 6. The opposite ends of the sheath 1 are closed by end plugs 5 and 7, respectively, which are welded to the sheath 1 in a gas tight manner. A narrow gap 8 is formed between the fuel pellets 2 and the inner wall of the sheath 1 for allowing the fuel pellets 2 to expand due to absorption of neutrons. As shown in FIG. 2, each fuel pellet generally comprises a sintered body of uranium oxide and generates a vast quantity of heat as a result of nuclear fission so that cracks 20 are often formed at random positions due to thermal stress. When cracked pieces of the fuel pellets enter into the gap 8, they are urged in the radial direction by the thermal expansion of the fuel pellet 2 thus damaging the sheath 1. In this manner, the damage of the sheath can not be completely prevented by merely providing gap 8. Although this difficulty can be obviated by increasing the width of the gap 8, too large a gap decreases heat conduction to the sheath 1. Although the sheath 1 has a high degree of ductility at the initial stage so that it can absorb local strains as the neutron irradiation proceeds its ductility decreases thus increasing fracture. Accordingly, the sheath 1 is liable to be damaged by the cracked pieces of the sintered fuel body. Further, the sheath is liable to be chemically corroded by the impurities contained in the fuel pellets or in the helium gas in the gap 8.
Among a plurality of pellets stacked in a sheath, those positioned at the axial center of the fuel rod have higher output peak than those positioned in the upper and lower regions. For this reason, it is desirable to make the gap between the sheath and the pellets contained in the central region of the sheath larger than the gap between the sheath and the pellets near the upper and lower ends.